In industrial process measurements technology, especially also in connection with the automation of chemical processes or procedures for producing a product from a raw or starting material by the use of chemical, physical or biological processes and/or the automated control of industrial plants, electronic measuring and/or switching devices installed near to too process, so called field devices are applied, which serve for producing measured values—analog or digital—representing process variables, as well as measured value signals bearing the measured values. Examples of such field devices include Coriolis mass flow measuring devices, density measuring devices, magneto inductive flow measuring devices, vortex flow measuring devices, ultrasonic flow measuring devices, thermal, mass flow measuring devices, pressure measuring devices, fill level measuring devices, temperature measuring devices, pH-value measuring devices etc. The respective process variables to be registered can include, depending on application, for example, a mass flow, a density, a viscosity, a fill level or a limit level, a pressure or a temperature or the like, of a liquid, powdered, vaporous or gaseous medium conveyed, respectively held, in a corresponding container, such as e.g. a pipeline or a tank.
For registering the respective process variables, electronic devices of the type being discussed have a corresponding physical to electrical or chemical to electrical, measuring transducer. This is most often installed in a wall of the container containing the medium or in the course of a line, for example, a pipeline, conveying the medium and serves to produce at least one electrical measurement signal corresponding to the process variable to be registered. For processing the measurement signal, the measuring transducer is further connected with an operating and evaluating circuit provided in a device electronics of the field device and serving for further processing or evaluating the at least one measurement signal, as well as also for generating corresponding measured value signals. Further examples of such measuring devices known, per se, to those skilled in the art, especially also details concerning their application and their operation, are set forth in, among others, German Patents: DE A 10 2005 025 670, DE A 10 2008 042972, DE A 101 26 654, and DE U 297 04 361[H]; USA 2004/0183550, USA 2006/0120054, US A 2006/0161359, US A 2009/0277278, U.S. Pat. No. 4,574,328, U.S. Pat. No. 4,850,213, U.S. Pat. No. 5,672,975, U.S. Pat. No. 5,706,007, U.S. Pat. No. 6,236,322, U.S. Pat. No. 6,366,436, U.S. Pat. No. 6,539,819, U.S. Pat. No. 6,556,447, U.S. Pat. No. 7,875,797; and published international applications WO A 02/103327, WO A 96/37764 and WO A 98/14763.
In the case of a large number of field devices, the measuring transducer is, for producing the measurement signal during operation, additionally so driven by a driver signal generated at least at times by the operating and evaluating circuit that it, in a manner suitable for the measurement, at least indirectly or, however, also via a probe directly contacting the medium, acts essentially directly on the medium, in order there to bring about reactions corresponding to the measured variable to be registered. The driver signal can be correspondingly controlled in such case, for example, as regards an electrical current level, a voltage level and/or a frequency. Examples of such active measuring transducers, thus measuring transducers correspondingly transducing an electrical driver signal in the medium, include, especially, the measuring transducers serving for measuring, at least at times, the flow of flowing media, e.g. measuring transducers with at least one coil producing a magnetic field driven by the driver signal, or with at least one ultrasound emitter driven by the driver signal, or, however, also fill level and/or limit level transducers serving for measuring and/or monitoring fill levels in a container, such as e.g. those with freely radiating microwave antenna, Goubau line or vibrating immersion element.
In the case of electronic devices of the type being discussed, the respective device electronics—most often embodied as a transmitter electronics, namely an electronics converting internal, proprietary, measured value signals into standardized, external, measured value signals, respectively telegrams—can usually be electrically connected via corresponding electrical lines to a superordinated electronic data processing system most often arranged spatially removed from the respective device and most often also spatially distributed, where the measured value signals correspondingly carrying the measured values produced by the respective device are forwarded near in time. Additionally, such devices during operation are usually connected with one another and/or with corresponding electronic process controllers by means of a data transmission network provided within the superordinated data processing system. The superordinated data processing system can be provided, for example, in the form of on-site, programmable logic controllers or process control computers installed in a remote control room, where the measured values, in given cases, produced by means of the device and digitized and correspondingly coded in suitable manner are forwarded. Such process control computers can further process the transmitted measured values and visualize them e.g. on monitors as corresponding measurement results and/or convert them into control signals for other field devices embodied as actuating devices, such as e.g. magnetic valves, electric motors, etc.
Since modern measuring arrangements formed by means of devices of the type being discussed are most often also directly monitored and, in given cases, controlled and/or can be configured from such control computers, in a corresponding manner, operating data intended for the respective field devices are equally transmitted via the aforementioned data transmission networks, which are most often hybrid as regards the transmission physics and/or the transmission logic.
Accordingly, the data processing system serves usually also to condition the measured value signal delivered, in given cases, by the field device, thus to condition it corresponding to the requirements of downstream data transmission networks, for example, suitably to digitize it and, in given cases, to convert it into a corresponding telegram, and/or to evaluate it on-site. For such purpose, there are provided in such data processing systems, electrically coupled using respective connecting lines, evaluating circuits, which pre- and/or further-process as well as, in case required, suitably convert, the measured values received from the respective electronic device formed, for instance, as a measuring and/or switching device. Serving for data transmission in such industrial data processing systems at least sectionally, especially serially, are fieldbusses, such as e.g. FOUNDATION FIELDBUS, RACKBUS-RS 485, PROFIBUS, etc., or, for example, also networks based on the ETHERNET standard, as well as the corresponding, most often comprehensively standardized, transmission protocols. Besides the evaluating circuits required for the processing and converting of the measured values delivered by the respectively connected field device, such superordinated data processing systems have most often also, for supplying the connected measuring and/or switching devices with electrical energy, electrical supply circuits, which provide a corresponding supply voltage, in given cases, fed directly from the connected fieldbus, for supplying power to the respective device electronics and the thereto connected electrical lines as well as driving the respective electrical currents flowing through the respective device electronics. A supply circuit can, in such case, be associated with, for example, exactly one field device and together with the evaluating circuit associated with the respective field device—for example, united into a corresponding fieldbus adapter—be accommodated in a shared electronics housing e.g. formed as a top hat rail module and/or installed in a circuitry cabinet.
In the case of devices of the type being discussed, consequently field devices of industrial measuring and automation technology, the particular device electronics is most often accommodated in a comparatively robust, for instance, impact-, pressure-, explosion- and/or weather resistant, electronics housing. This can be arranged remotely from the device and be connected with such only via a flexible line; it can, however, also, such as shown e.g. in the initially mentioned disclosures or that shown here, be arranged directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer. Examples of such electronics housings suitable for field devices are described in, among others, the initially mentioned U.S. Pat. No. 6,366,436, German Patents DE A 101 26 654, and DE A 10 2008 042 972, U.S. Pat. No. 6,556,447 or international published application WO A 98/14763. In accordance therewith, such electronics housings comprise a most often pot-shaped housing basic body having one or more cavities. Laterally bounding the cavity is a most often sectionally circularly cylindrical, side wall having an open end and a rear wall bounding the cavity on an end oppositely lying and remote from the open end, for example, a flat or outwardly bulged, rear wall, in given cases, also a releasable, rear wall, as well as a housing cap releasably connected with the housing basic body on its open end, for example, by means of screwed connection, and serving as a closure for the basic body. The housing cap, which has most often also a viewing window integrated therein—for example, a viewing window enabling observation of a display element placed therebehind within the housing basic body—usually has a screwed connection with the housing basic body, for example, in the manner of a screw cap closure. Said viewing window is, in such case, most often formed by means of a translucent, respectively transparent material, such as, for instance, a glass, glass ceramic or plastics material, most often in the form of a window pane, which covers a window-opening provided in a most often metal or at least metallized, cap basic body. The opening is usually formed in a cap floor of the basic cap body, while the window pane is placed on a side of the cap floor facing the cavity and affixed there, and, indeed, in such a manner that the window pane closes the opening.
Industrial-strength electronic devices, consequently also their particular electronics housing and their therein respectively accommodated device electronics, must, as is known, satisfy very high protection requirements especially as regards the shielding of the therein placed electrical components against external, environmental influences, as regards protection against possible touching of voltage-carrying components and/or as regards suppression of electrical ignition sparks in the case of malfunction. For this, there exists, for example, also the requirement that an electrical current, which could flow, for example, in the case of the electronics housing being in contact with an internal voltage, via the electronics housing to ground or earth, not exceed a maximum allowable value. In the case of a connection of the electrical device to 250 V, this maximum allowable value amounts to, for example, 10 mA. If these requirements are fulfilled, then the device meets at least the requirements of protection class 11, i.e. it is an electrical device with protective insulation. For implementing these requirements, it is accordingly required that the housing of the electrical device be sufficiently insulated relative to all voltage-carrying parts of the device. Such insulation is especially necessary when the housing is composed, of electrically conductive material, for example, a metal. Moreover, electronics housings, thus the device electronics placed therein, must be sufficiently protected against penetration of moisture or impurities, especially dust, as well as contact from the outside. The degree of protection to be fulfilled on the part of the respective electronics housing, not least of all also with respect to the given application and environmental conditions, against penetration, for instance, by contact of foreign particles, respectively water, is determinable, for example, based, on the protection types defined in the German standards DIN EN 60529, respectively DIN 40 050, e.g. “dust, respectively water spray, protected from all sides (IP54)” or “dust, respectively ongoing immersion, tight (IP68)”, respectively classes, or according to industrial standard NEMA 250.
Electronic devices, consequently also field devices, which also might be operated in explosion-endangered regions, must, moreover, also satisfy very high safety requirements as regards explosion protection. In such case, of concern, especially, is safely preventing the formation of sparks or at least assuring that a spark possibly occurring in the interior of a closed space has no effects on the surroundings, in order so safely to avoid the potentially possible triggering of an explosion.
As explained for this, for example, also in the initially mentioned U.S. Pat. No. 6,366,436 and U.S. Pat. No. 6,556,447, different ignition protection types are distinguished, which are correspondingly manifested in standards relevant for electrical operating means for explosion-endangered regions, among these being the European standards EN 60079-xx, the US-American standards FM36xx, the Canadian standard C22.2, the international standard IEC 60079-18 or the standards DIN EN 50 014 ff. Thus, e.g. according to the European standard EN 60079-11:2007, explosion protection is present when electronic devices are embodied according to the therein defined ignition protection type or also the protection class with the name “Intrinsic Safety” (Ex i). In this protection class, the values for the electrical variables, electrical current, voltage and power in an electronic device must, at all times, in each case, lie below a predetermined limit value. The three limit values are so selected that, in the case of malfunction, e.g. due to a short-circuit, the maximum occurring energy is not sufficient to produce an ignition spark. The electrical current is kept, e.g. by resistances, the voltage e.g. by Zener diodes and the power by corresponding combinations of electrical current and voltage limiting components, below the predetermined limit values. European standard EN 60079-7:2007 provides another protection class with the name “Increased Safety” (Ex e). In the case of electronic devices, which are embodied according to this protection class, the ignition, respectively explosion, protection is achieved by making the spatial distances between two different electrical potentials so large that a spark formation can also not occur, due to the distance, in the case of malfunction. This can, however, in given cases, lead to the fact that circuit arrangements must have very large dimensions, in order to satisfy these requirements. Another protection class is that of European standard EN 60079-1:2007, namely ignition protection type “Pressure Resistant Encapsulation” (Ex d). Electrical devices embodied according to this protection class must have a pressure-resistant housing, in order to assure that an explosion occurring in the interior of the housing cannot be transmitted into the surrounding space.
Pressure resistant housings are embodied with comparatively thick walls, in order that they have sufficient mechanical strength. Another standard, namely EN 60079-18:2004, relates to the protection class “Potting Encapsulation” (Ex m). In such case, of concern is an ignition protection type, in which components and/or assemblies of the electronic device, which could potentially ignite an explosive atmosphere through sparks or through warming, are encapsulated in a most often elastomeric and/or foamed, embedding compound of synthetic material, in order largely to exclude contact with an explosion-endangered atmosphere and so to avoid ignition. In the USA, in Canada, in Japan and in other countries, standards comparable with the aforementioned European standards exist, for instance, FM3600, FM3610, respectively C22.2 No. 157, etc.
Furthermore, electronic devices of the type being discussed are also subject to requirements concerning electromagnetic compatibility (EMC), respectively they must satisfy corresponding testing standards.
In the case of housing caps with a therein integrated, viewing window, such forms a weak point of the total electronics housing, since a connection between window pane and cap basic body sufficiently sealed against the penetration of moisture, respectively water, can only be manufactured with comparatively great effort, for instance, by applying an adhesive between window pane and cap basic body. Said connection, as well as also the window pane, must additionally also fulfill requirements as regards impact, breaking, respectively explosion resistance, as well as also, at all times, the required sufficient blocking, respectively shielding, action against electromagnetic waves, consequently be able to exhibit the required electromagnetic compatibility.